This is the move which initiates the Kalashnikov variation. Black allows White to establish a highly restrictive 'bind' on the centre, relying on the solidity of his position and planning to gain counterplay later on by opening up the queenside. The pawn structure (Black pawns on d6 and e5 in the Sicilian) bears some relation to the Najdorf variation, but in the Najdorf White is not given the chance to lock down the central pawn structure as follows:

6.c4 Be7
7.N1c3 a6
8.Na3 Be6

Even though Black's backward pawn on d6 is theoretically weak, his pieces can defend it very well, and in fact White does not usually focus his strategy on this obvious weakness, preferring instead to try and build up in the centre and slowly squeeze the Black position, sometimes opting for a kingside attack.